A CSD system of the general type referred to herein for use in aircraft installations is shown in U.S. Pat. No. 3,733,924. The system disclosed therein employs a mechanical differential in conjunction with a hydrostatic transmission to rotate an aircraft alternator at constant speed via power supplied from a variable speed aircraft engine. The hydrostatic transmission is associated with a closed hydraulic circuit. Fluid is supplied to the circuit through a charge pump from a sump. Fluid pressure is regulated by a charge relief valve. An external circuit is provided for the IDG. However, this system does not address the problem of how to refill external cooling lines connected to the CSD to make up for lost fluid so that the lines are filled with fluid while being completely purged of air.
Another aircraft CSD system is shown in U.S. Pat. No. 3,600,106. This system aims at reducing the required component weight in aircraft by providing a sump pressurization approach which eliminates the need for external engine bleed hardware. This aim is achieved by supplying air-entrained liquid into the transmission sump to attain a sufficient inlet head for the scavenge pump to maintain a desired efficiency level. A relief valve maintains a preset charge pressure in the fluid circuit. A differential pressure valve is used to maintain a sufficient pressure within a transmission sump so that there is sufficient pressure head at the inlet of a scavenge pump used for drawing leaking fluid from the transmission sump and delivering that fluid to the return conduit.
A charge pump is utilized to provide makeup, lubricating and control fluid for the system. The charge or makeup fluid is fed through a conduit connected to a port in a hydraulic drive to make up for loss of fluid caused by leakage. Excess charge fluid flows over a charge relief valve which maintains a preset charge pressure in the conduit. A sump is provided in the case of the CSD to collect leakage fluid which is then pumped by means of a scavenge pump to a return conduit which passes through an external cooling circuit, i.e. the cooler is located outside the case or housing of the CSD system, and then to a reservoir where air entrained in the fluid is separated before the fluid is returned to the charge pump. However, no provision is made in this system for filling the conduits in the external cooling circuit and the rest of the system with fluid and completely purging the circuit of air during the filling process when the system is not in operation.
Valves with movable spools have long been known for variety of uses. For example, U.S. Pat. No. 4,537,284 discloses a progressive distributor valve for distributing lubricant to heddles of weaving machines. However, this valve is not designed for the purpose of pressurizing an oil circuit at the same time that it is being filled. Also, there is no concern about purging air from the lubricant inasmuch as the lubricant is discharged through channels into an air space in the form of heddle guide grooves.
U.S. Pat. No. 3,964,506 shows a pressure control system which provides for rapid filling and emptying of a clutch cylinder during engagement and disengagement of the clutch. The system uses a relief type pressure regulating valve, a solenoid-operated pilot valve and a flow control valve. However, these types of valves are not intended to operate as service valves strictly by pressure actuation and so as to provide their own pressure regulation when the system is not operating.
U.S. Pat. No. 3,995,727 shows the use of a valve to provide additional cooling to the clutch of an automatic transmission. However, it will be readily appreciated that the arrangement and activation of this valve is not at all useful for service valve applications requiring high pressure fill and air purge capability.